bigint: reapply #176
bigint: reapply #176

koverstreet/num,vks/num,rust-lang/num,rust-num/num-integer,DonSheddow/num,koverstreet/num,rust-num/num-bigint,rust-lang/num,vks/num,rust-num/num-integer,rust-num/num,DonSheddow/num,rust-num/num-traits,rust-num/num-bigint,rust-num/num,rust-num/num-traits